It is well known in the art that the resistance modulation of Hall elements or magnetoresistors can be employed in position and speed sensors with respect to moving magnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object rotating relative, and in close proximity, to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the rotating target wheel is adjacent to the MR than when a slot of the rotating target wheel is adjacent to the MR. The use of a constant current excitation source provides an output voltage from the MR that varies as the resistance of the MR varies.
Accurate engine crank position information is needed for ignition timing and OBDII mandated misfire detection. Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position information during cranking. Various combinations of magnetoresistors and single and dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 5,570,016, 5,714,883, 5,731,702, and 5,754,042).
The crank position information is encoded on a rotating target wheel in the form of teeth and slots. The edges of the teeth define predetermined crank positions. The sensor is required to detect these edges accurately and repeatably over a range of air gaps and temperatures. Virtually all such sensors are of the magnetic type, either variable reluctance or galvanomagnetic (e.g., Hall generators or magnetoresistors). Galvanomagnetic sensors are becoming progressively most preferred due to their capability of greater encoding flexibility and speed independent output signals.
Furthermore, temperature and the size of the air gap affect the output signal of a magnetic sensing element. Consequently, operation over wide temperature and air gap size ranges requires some form of compensation for the resultant signal drift, both in amplitude and offset. The most common approach is the use of two matched sensing elements operating in a differential mode thereby providing a common mode rejection.
High accuracy and repeatability magnetic position sensors employ two matched sensing elements such as magnetoresistors (MR) or Hall generators. They are spaced a few mm apart from each other, either in the axial direction (dual track target wheels) or along the target periphery (sequential sensors). The primary purpose of using two matched sensing elements is common mode signal rejection, since the sensing elements are equally affected by temperature and air gap. Having perfectly matched sensor elements, however, is not sufficient. The uniformity of the bias magnet, packaging tolerances, and inaccuracies of sensor installation can introduce unknown offsets to the output signals of the sensing elements. Presently, selection of matched MR pairs, a tight process control during all phases of sensor manufacture with a final testing of each sensor, is employed to build sensors meeting the required specifications. Unfortunately, this approach increases the final cost of the sensor.
Angular position information is contained in the location of target wheel tooth edges (i.e., tooth/slot transitions), and at these locations the output signals of the MRs are by design unequal so that their differential signal is nonzero. Over a slot or tooth, both MR output signals should be equal so that their differential signal is zero but, frequently, the MRs are not well matched resulting in a nonzero differential signal causing an erroneous output signal and switching leading to an incorrect crank position and speed of rotation.
An example of such a sensor is the sequential crankshaft sensor used on several of General Motors Corporation trucks. This sensor employs two InSb magnetoresistor elements located radially proximate to the target wheel, one being slightly displaced with respect to the other in the direction of target wheel rotation. FIG. 1 is a schematic representation of an exemplar automotive environment of use according to this prior art scheme, wherein a target wheel 10 is rotating, such as for example in unison with a crankshaft, a drive shaft or a cam shaft, and the rotative position thereof is to be sensed. Rotative position of the target wheel 10 is determined by sensing the passage of a tooth edge 12, either a rising tooth edge 12a or a falling tooth edge 12b, using a single dual MR differential sequential sensor 14. A tooth edge 12 is considered rising or falling depending upon the direction of rotation of the target wheel 10 with respect to the magnetoresistive sensors MR1 and MR2. MR1 is considered leading and MR2 is considered lagging if the target wheel 10 is rotating in a clockwise (CW) direction whereas if the target wheel is rotating in a counterclockwise (CCW) direction then MR1 is considered lagging whereas MR2 is considered leading. For purposes of example, the target wheel 10 will be assumed to be rotating in a CW direction in the views.
The single dual MR differential sequential sensor 14 employs two magnetoresistor elements, MR1 and MR2, which are biased by a permanent magnet 16, wherein the magnetic flux 18 and 20 emanating therefrom are represented by the dashed arrows. The magnetic flux 18 and 20 pass from the permanent magnet 16 through the magnetoresistors MR1 and MR2 and through the air gaps 22 and 24 to the target wheel 10. The target wheel 10 is made of a magnetic material having teeth 26 and spacings 28 therebetween. The spacing L between MR1 and MR2 is generally such that the trigger points for the rising and falling edges of the output signal V.sub.OUT are dependent on the leading MR only, as will be later described.
Power V.sub.IN is supplied to CURRENT SOURCE1 30 and CURRENT SOURCE2 32 through voltage source 34. Power is also supplied to a comparator 36 (with hysteresis) through voltage source 34, but is not shown. CURRENT SOURCE1 30 supplies current to MR1 thereby providing for an output voltage V.sub.MR1 from MR1. CURRENT SOURCE2 32 supplies current to MR2 thereby providing for an output voltage V.sub.MR2 from MR2. Output voltages V.sub.MR1, and V.sub.MR2 are input into the comparator 36 whose output voltage V.sub.OUT is an indication of the position of rotation of the target wheel 10. It is to be understood that all voltages are measured with respect to ground unless otherwise indicated herein, and that CURRENT SOURCE1 30 is matched to CURRENT SOURCE2 32.
In a first example, wherein the two MR elements are matched, as shown in FIG. 2A, the lagging MR element, in this case MR2, provides a delayed signal in every respect identical to the signal from the leading MR, in this case MR1. The differential signal V.sub.D =V.sub.MR1 -V.sub.MR2, shown in FIG. 2B is electronically generated within the comparator 36 and is then used by the comparator to reconstruct the signal V.sub.OUT (shown in FIG. 2C) emulating the profile of the target wheel 10. Upon a closer inspection of FIGS. 2A, 2B and 2C, it becomes evident that the rising edges 42 and the falling edges 44 of the sensor output signal V.sub.OUT are determined only by first points 46 corresponding to the rising edges and second points 48 corresponding to the falling edges where the signal from the leading MR, in this example MR1, crosses a first threshold voltage 50 corresponding to the first points and a second threshold voltage 52 corresponding to the second points wherein the first and second threshold voltages are determined by the hysteresis applied to the comparator 36. The lagging MR, in this example MR2, has no part in the generation of the rising edges 42 or the falling edges 44 of the output signal V.sub.OUT. The lagging MR, MR2, in this example, has the same offset voltage 54 as the leading MR, MR1, thereby leading to a zero voltage difference in the differential signal V.sub.D =V.sub.MR1 -V.sub.MR2 whenever MR1 and MR2 are not adjacent to or in close proximity to a rising tooth edge 12a or a falling tooth edge 12b of the target wheel 10 due to the matching of the MRs as depicted by signal line 54a in FIG. 2B.
As previously stated, over a slot or tooth, both MR output signals should be equal so that their differential signal is zero but, frequently, the MRs are not well matched resulting in a nonzero differential signal causing switching errors and an erroneous output signal leading to incorrect crank positions and speeds of rotation. Mismatch of the MRs can occur due to offset differences resulting in different bias voltages for each MR or due to gain (sensitivity) differences resulting in different signal amplitudes for each MR or due to a combination of offset and gain differences between the MRs.
FIG. 3A is a second example of a schematic representation of an exemplar automotive environment of use according to this prior art scheme wherein the two MR elements of the sensor, configured as in FIG. 1, are mismatched due to a gain error wherein MR1 has a lower voltage V'.sub.1H over a tooth 26 than the voltage V'.sub.2H over the same respective tooth produced by MR2 whereas both MRs have the same voltage V.sub.12 over a slot 28 of the target wheel 10.
In this second example, as shown in FIG. 3A, the lagging MR element, in this case MR2, provides a delayed signal having a voltage gain offset V'.sub.2H -V'.sub.1H over a tooth 26. The differential signal V'.sub.D =V'.sub.MR1 -V'.sub.MR2, shown in FIG. 3B is electronically generated within the comparator 36 and is then used by the comparator to reconstruct the signal V'.sub.OUT (shown in FIG. 3C) which should emulate the profile of the target wheel 10. Upon a closer inspection of FIGS. 3A, 3B and 3C, it becomes evident that the rising edges 42' of the sensor output signal V'.sub.OUT are determined only by first points 46' corresponding to the rising edges where the signal from the leading MR, in this example MR1, crosses a first threshold voltage 50' corresponding to the first points wherein the first threshold voltage is determined by the positive hysteresis +.DELTA.V' applied to the comparator 36 whereas the falling edges 44' of the sensor output signal V'.sub.OUT are determined only by second points 48' corresponding to the falling edges where the signal from the lagging MR, in this example MR2, crosses a second threshold voltage 52' corresponding to the second points wherein the second threshold voltage is determined by the negative hysteresis -.DELTA.V' applied to the comparator.
However, in this particular example, due to the voltage gain error there is a nonzero voltage difference in the differential signal V'.sub.D =V'.sub.MR1 -V'.sub.MR2 whenever MR1 and MR2 are adjacent to or in close proximity to a tooth 26 of the target wheel 10 due to the mismatching of the MRs as depicted in FIG. 3B by voltage level V'.sub.DL wherein this example V'.sub.DL is at a lower voltage level than the negative hysteresis -.DELTA.V' applied to the comparator 36. As shown in FIGS. 3B and 3C, this results in switching errors causing an erroneous output voltage V'.sub.OUT which does not emulate the profile of the target wheel 10.
FIG. 4A is a third example of a schematic representation of an exemplar automotive environment of use according to this prior art scheme wherein the two MR elements of the sensor, configured as in FIG. 1, are mismatched due to an offset error wherein MR1 has a higher bias voltage V".sub.MR1 over a tooth 26, represented by V".sub.1H, and a slot 28, represented by V".sub.1L, than the bias voltage V".sub.MR2 over the same respective teeth, V".sub.2H, and slots, V".sub.2L, produced by MR2.
In this third example, as shown in FIG. 4A, the lagging MR element, in this case MR2, provides a delayed signal in every respect identical to the signal from the leading MR, in this case MR1, except for the bias voltage offset V".sub.1H -V".sub.2H over a tooth 26 and V".sub.1L -V".sub.2L over a slot 28. The differential signal V".sub.D =V".sub.MR1 -V".sub.MR2, shown in FIG. 4B is electronically generated within the comparator 36 and is then used by the comparator to reconstruct the signal V".sub.OUT (shown in FIG. 4C) which should emulate the profile of the target wheel 10. Upon a closer inspection of FIGS. 4A, 4B and 4C, it becomes evident that the rising edges 42" of the sensor output signal V".sub.OUT are determined only by first points 46" corresponding to the rising edges where the signal from the leading MR, in this example MR1, crosses a first threshold voltage 50" corresponding to the first points wherein the first threshold voltage is determined by the positive hysteresis +.DELTA.V" applied to the comparator 36 whereas the falling edges 44" of the sensor output signal V".sub.OUT are determined only by second points 48" corresponding to the falling edges where the signal from the lagging MR, in this example MR2, crosses a second threshold voltage 52" corresponding to the second points wherein the second threshold voltage is determined by the negative hysteresis -.DELTA.V" applied to the comparator.
However, in this particular example, the lagging MR, MR2, has an offset voltage 54b" which is not the same as the offset voltage 54a" of the leading MR, MR1, thereby leading to a nonzero voltage difference in the differential signal V".sub.D =V".sub.MR1 -V".sub.MR2 whenever MR1 and MR2 are not adjacent to or in close proximity to a rising tooth edge 12a or a falling tooth edge 12b of the target wheel 10 due to the mismatching of the MRs as depicted in FIG. 4B by voltage level V".sub.DH wherein this example V".sub.DH is at a higher voltage level than the positive hysteresis +.DELTA.V" applied to the comparator 36. As shown in FIGS. 4B and 4C, this results in switching errors causing an erroneous output voltage V".sub.OUT which does not emulate the profile of the target wheel 10.
What is needed is a method and apparatus wherein continuous adaptive matching of both MR output signals during sensor operation of a single dual element sensor, preferably, but not exclusively, a single dual element magnetoresistive sensor, is utilized to sense crankshaft position and rotational speed from the passage of single tooth edges of an encoder or target wheel.